Some of our optionally-run build-time tests use #!/usr/bin/env python, and we actually run them under PYTHON as auto-detected by configure.ac. However, they use Python 2 syntax.
This is not going to work if PATH contains python3 but not python (which turns out to be the case for current Debian autobuilders), or if /usr/bin/python is Python 3 (which is a terrible idea[1], but Arch Linux does it anyway).
It turns out this is trivial to fix.
[1] https://www.python.org/dev/peps/pep-0394/